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Mr Kerr said examples included those working at car washes and in construction as well as in agriculture and food processing - often receiving very little pay and forced to put up with poor living conditions.

A cannabis factory in Cambridgeshire where slave labour was allegedly used

Others sold into slavery could be kept in pop-up brothels, where sex workers promised a better life are left penniless with few clothes other than underwear, while some can be working at cannabis factories, he added.

He said: "As you go about your normal daily life and as you're engaged in a legitimate economy accessing goods and services, there is a growing and a good chance you will come across a victim who has been exploited in one of those different sectors.

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But he also cited one example of a 12-year-old Roma girl being stopped at border control, bound for a life as a domestic slave.

He said: "She was being brought in to work for a family in part of the UK, where she had effectively been sold by her father - or it had been facilitated by her father - and she was being brought in to take this family's children to school and pick them up every day, and clean the house in between. She was 12 years old, same age as my youngest son."

Mr Kerr said criminal charges were pending against those involved in the case.

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He added: "People are being exploited on an hourly and daily basis. The full scale and extent of it, we don't know. But what we have found is that in every medium-to-large town and every city in the UK, we have found evidence of vulnerable people being exploited.

"We can't put a figure on it but we can say there are tens of thousands of victims across the UK."

Earlier this year, the NCA released figures which showed the number of suspected victims of slavery and human trafficking had more than doubled in three years.

There were 3,805 people reported as potential victims in 2016, an increase from 1,745 in 2013, according to NCA statistics.